(i) Field of the Invention
This invention relates to the production of chlorine dioxide, and more particularly to an improved integrated process and integrated system for producing chlorine dioxide continuously, efficiently and rapidly with high yields.
(ii) Description of the Prior Art
Chlorine dioxide has been industrially employed as a bleaching agent by the cellulose pulp industry for over half a century. The industrial production of chlorine dioxide has grown substantially over the years.
The demand for chlorine dioxide is projected for significant growth in the next decade because many pulp mills are committed towards substitution of chlorine through the use of chlorine dioxide. This substitution is a result of new regulations worldwide limiting the pulp mills effluent of chlorinated organics. In addition, the delignification and bleaching of pulp should be carried out without the production of chloroform, furans and dioxins.
This substitution of chlorine by chlorine dioxide represents an increased cost to pulp mills due to the higher cost of an equivalent amount of chlorine dioxide. Furthermore, the conventional chlorine dioxide plants yield, as by-products, spent acid, salt cake and sodium chlorate. This acid solution, and/or slurry (if the salt cake is crystallized) is undesirable to pulp mills because, when fed to the chemical recovery system, it can be the cause of production down time and maintenance costs associated with the boiler tubes.
In the well-known process for generating chlorine dioxide, sodium chlorate reacts with hydrogen chloride in an aqueous solution to yield chlorine dioxide, chlorine, and sodium chloride. The chemical expression of this reaction may be written: EQU NaClO.sub.3(aq) +2HCl.sub.(aq) .fwdarw.ClO.sub.2 +NaCl.sub.(aq) +H.sub.2 O+1/2Cl.sub.2 ( 1)
This chemical process also has a waste reaction, a reaction that consumes sodium chlorate without making chlorine dioxide. The chemical expression for this reaction may be written: EQU NaClO.sub.3(aq) +6HCl.sub.(aq) .fwdarw.NaCl.sub.(aq) +3H.sub.2 O+3Cl.sub.2( 2)
The efficiency of the reaction is the fraction of a mole of chlorine dioxide generated per mole of sodium chlorate consumed. If one whole mole of chlorine dioxide is generated per mole of sodium chlorate consumed, the efficiency is 100%. By this definition, reaction is more efficient when the first reaction is more dominant. Also, where this reaction is less efficient, the ratio of chlorine generated to chlorine dioxide generated increases.
Efficiency is a complex function of concentration and temperature. Generally, the first reaction is more dominant at lower temperatures (which means higher efficiency). High sodium chlorate concentrations seem to favour high efficiency.
The production rate is also a complex function of concentration and temperature. Generally, the reaction is faster at higher temperatures (which means faster production rates). Generally, the reaction is slower at lower concentrations (which means slower production rates).
To minimize reaction (2), it has been suggested to react properly proportioned mixtures of chlorates, chlorides and a strong inorganic acid in strong solutions (containing at least 50% and up to 75% of water) at temperatures below 60.degree. C. Based on reaction (1), equivalent ratios of Cl.sup.- /ClO.sub.2 =2 and of H.sup.+ /ClO.sub.3.sup.- =2 should give high yields of ClO.sub.2 per mole of chlorate decomposed. In practice, however, it has been proposed in particular to use a ratio of H.sup.+ /ClO.sub.3.sup.- in excess of 2 because reaction (2) uses some of the chlorate in producing chlorine instead of ClO.sub.2. This proposal results in the use of excessive quantities of reactants.
Since chlorine dioxide is extremely explosive at high temperatures, the reactions described hereinabove have generally been carried out at relatively low temperatures. Furthermore, in order to reduce still further the danger of explosion, a non-reactive (inert) gas has generally been conducted into the reaction vessel. The purpose of the gas was to reduce the concentration of chlorine dioxide in the vessel to a nonexplosive proportion. In processes where gaseous hydrogen chloride was used or where large amounts of inert gases were used, there was a need for a rather large compressor. The need for such compressor involved increased capital and operating costs.
Furthermore, it has been recognized that a high yield of ClO.sub.2 per mol of chlorate decomposed, while desirable, is not alone sufficient to make the process economical for large scale production of chlorine dioxide. As a matter of practical necessity, it has therefore been recommended that the decomposition of the chlorate initially present be carried substantially to completion to avoid any appreciable waste of this valuable raw material. However, the requirement of consuming all, or almost all, of the chlorate entails inherent difficulties which greatly decrease the efficiency, rapidly and therefore the economy of the older process. One difficulty is the fact that the average hourly output of ClO.sub.2 is necessarily low because the reaction rate decreases considerably as the concentration of the reactants, particularly of chlorate, decreases. The use of solutions of low chlorate content further magnifies this effect and wastes valuable space in the reaction chamber. Finally, as the concentration of chlorate decreases, reaction (2) contributes increasingly to the decomposition of the chlorate whereby the overall yield of chlorine dioxide is lowered.
There are many patents directed to the preparation of chlorine dioxide by introducing an aqueous solution of sodium chlorate and sodium chloride and an acidic agent into a reaction vessel in a continuous manner and by way of an integrated process and integrated apparatus.
An integrated chlorine dioxide plant is a plant that has four main process systems:
one process system is the generator. This system generates chlorine dioxide from strong brine and hydrochloric acid. Chlorine dioxide leaves the generator system as part of a gas mixture containing chlorine and water vapour. The strong brine is converted to weak brine as it passes through the generator system.
Another process system is the electrolysis system. This system receives weak brine from the generator system, strengthens the sodium chlorate concentration in the brine, and returns the strong brine to the generator system. This system produces a hydrogen gas co-product.
Another process system is the hydrochloric acid synthesis system. This system combines hydrogen gas from the electrolysis system with fresh chlorine and recycled chlorine to make hydrogen chloride. The hydrogen chloride is absorbed into water to make hydrochloric acid. Hydrochloric acid is forwarded to the generator system.
The remaining process system is the chlorine dioxide absorption system. This system draws the gas mixture from the generator system and contacts the it with water. Chlorine dioxide, water vapour, and some of the chlorine in the gas mixture are absorbed into water solution. The chlorine dioxide solution is forwarded to its end use. The unabsorbed chlorine is compressed and sent to the hydrochloric acid synthesis system as recycle chlorine.
The integrated plant places specific demands on the generating system. The generating system produces weak brine that flows back to the electrolysis system. It is important that the residual hydrogen chloride in the weak brine be low enough so that it does not interfere with the operation of the electrolysis system. If the hydrogen chloride concentration is too great, the electrolysis system will generate excess chlorine and electrolysis efficiency will be impaired. Even worse, the excess chlorine will be evolved in the hydrogen gas stream, thus creating an explosion hazard. Another demand placed on the generating system is maintaining the water balance in the strong brine/weak brine loop. Excess water enters the strong brine/weak brine loop from the water content of the hydrochloric acid stream. The water balance is maintained by operating the generating system to evaporate the excess water.
The pulp and paper industry is the main market for large chlorine dioxide generators. That industry uses chlorine dioxide to bleach pulp so that white paper can be made from it. Because of environmental concerns, the pulp and paper industry prefers that chlorine dioxide be delivered to their bleach process with a minimum of the chlorine co-product. Therefore, it is important that chlorine dioxide generators be efficient to minimize the cost of equipment and resources required to separate chlorine from the chlorine dioxide product.
Among the patents are the following:
Canadian Patent 461,586 patented Dec. 6, 1949 by G. A. Day which provided a process for the manufacture of chlorine dioxide by supplying an aqueous solution of an inorganic chlorate to the reaction chamber, and gaseous hydrogen chlorides The hydrogen chloride was supplied to the chamber in an amount insufficient to react with all the chlorate present therein. The resulting acid was reacted with the chlorate. Gaseous chlorine dioxide and chlorine were removed from the reaction chamber. Partially-reacted chlorate solution from the reaction chamber was passed to an electrolytic chlorate cell to increase the chlorate content thereof. The fortified chlorate solution was returned to the reaction chamber for further reaction with hydrogen chloride. The low amount of chloride in the electrolytic chlorate liquor did not always serve to prevent the precipitation of chloride salts during the primary reaction to form the chlorine dioxide. Such precipitation of chloride salts reduced the efficiency of the production of chlorine dioxide generator.
Canadian Patent 782,574 patented Apr. 9, 1968 by G. O. Westerlund, which provided an improved continuous recyclic process and apparatus for the production of chlorine dioxide. That patented process involved the first step of effecting electrolysis of an aqueous solution of a metal chloride to form an aqueous solution of a metal chlorate and gaseous hydrogen. The gaseous hydrogen was reacted with gaseous chlorine to form gaseous hydrogen chloride. The aqueous solution of metal chlorate was reacted with the gaseous hydrogen chloride to form an aqueous solution of metal chloride, which was recycled to the first step and an aqueous solution of chloric acid. The aqueous solution of chloric acid was reacted with the gaseous hydrogen chloride to form chlorine dioxide, which was recovered, water and gaseous chlorine, which was recycled to the second step. Such process was thus based on a system which required water, chlorine and electric current for the production of chlorine dioxide. This process suffered from the problem that the ratio of ClO.sub.2 to Cl.sub.2 produced from the absorber was not controlled to be sufficiently high.
Canadian Patent No. 956,783 patented Oct. 29, 1979 by D. G. Hatherly, provided a method of forming chlorine dioxide which included forming an aqueous reaction medium containing an alkali metal chlorate and hydrochloric acid in a reaction zone. The aqueous reaction medium was heated preferably to its boiling temperature while the reaction zone was maintained under a reduced pressure to prevent spontaneous decomposition of chlorine dioxide as it was evolved. This reaction generated chlorine dioxide and chlorine, and evaporated water from the medium. The chlorine dioxide and chlorine were removed from the reaction zone along with the evaporated water. The aqueous reaction medium was formed by the steps of electrolyzing an aqueous solution of an alkali metal chloride to form an aqueous solution of an alkali metal chlorate and hydrogen, then feeding the aqueous solution of the alkali metal chlorate to the reaction zone, then forming hydrogen chloride by reaction between approximately one-third of the whole amount of the hydrogen and chlorine, and finally feeding the hydrogen chloride to the reaction zone.
Canadian Patent No. 956,784 patented Oct. 29, 1974 by J. D. Winfield, provided a process for the preparation of chlorine dioxide. The patented process involved forming an aqueous reaction medium containing an alkali metal chlorate and hydrochloric acid in a reaction zone by feeding hydrochloric acid and an aqueous solution of the alkali metal chlorate to the reaction zone. The aqueous reaction medium was heated to effect reaction between the alkali metal chlorate and hydrochloric acid to generate chlorine dioxide and chlorine and to evaporate water from the medium under reduced pressure. The chlorine dioxide and chlorine were removed from the reaction zone as a gaseous mixture consisting of chlorine dioxide and chlorine and evaporated water.
Canadian Patent No. 976,726 patented Oct. 28, 1975 by G. Cowley provided a continuous method of producing chlorine dioxide which included continuously maintaining a chlorine dioxide-producing reaction medium in a reaction zone. The reaction medium contained an alkali metal chlorate, a reducing agent capable of reducing the alkali metal chlorate to chlorine dioxide and chlorine and a strong mineral acid. Chlorine dioxide and chlorine were continuously generated from the reaction medium and water was continuously evaporated from the reaction medium at substantially the boiling point thereof while the reaction medium was maintained under a reduced pressure. A gaseous mixture consisting of the generated chlorine dioxide and chlorine and the evaporated water was removed from the reaction zone. Chlorine dioxide was continuously recovered from the gaseous mixture. An alkali metal salt of the anion of the strong acid was continuously depositing in the reaction zone. A slurry containing deposited alkali metal salt and part of the reaction medium was continuously removing from the reaction zone. A recycle mixture having a reduced alkali metal salt solids content and containing make-up quantities of alkali metal chlorate and reducing agent, except that the reducing agent was omitted from the recycle mixture when the strong mineral acid also provides the reducing agent was continuously formed. The recycle mixture was heated to the boiling point of the reaction medium at the prevailing absolute pressure in the reaction zone. The heated mixture was accelerated to establish a back pressure exceeding the difference in saturation vapour pressure of the heated mixture and of the reaction medium. A strong mineral acid was added to the accelerated beaten mixture at the maximum velocity of the mixture resulting from the acceleration to provide a feed material for the reaction zone. Gaseous material in the feed material was allowed to expand while a low rate of acceleration was maintained. The expanded feed mixture was fed to the reaction zone at a level above the liquid level in the reaction zone. The level of liquid in the reaction zone was maintained substantially constant. A problem with this patented process is the disposal of the precipitated salts.
Canadian Patent No. 1,049,950 patented Mar. 6, 1979 by J. D. Winfield, provided an integrated process for the production of chlorine dioxide. Such integrated process involved providing an aqueous acid reaction medium containing sodium chlorate and hydrochloric acid in the reaction zone. The sodium chlorate was continuously in reduced with the hydrochloric acid predominantly accordance with the equation: EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O+NaCl
thereby continuously generating chlorine dioxide and chlorine, while simultaneously continuously evaporating water from the aqueous reaction medium as a gaseous mixture of chlorine dioxide, chlorine and steam from the reaction zone. The reaction zone was continuously maintained under a reduced pressure. Sodium chlorate and hydrogen chloride were continuously added to the reaction medium at rates equivalent to their rates of consumption to form chlorine dioxide and chlorine. The water was evaporated from the reaction medium at a rate sufficient to maintain a substantially constant volume of aqueous reaction medium in the reaction zone. Sodium chloride was removed from the reaction zone, either in solid form or in solution in the aqueous reaction medium. An aqueous solution of the removed sodium Chloride was continuously electrolyzed to form an aqueous solution of sodium chlorate and .gaseous hydrogen in accordance with the equation: ##STR1## That aqueous solution of sodium chlorate was fed to the reaction medium, the rate of production of sodium chlorate from sodium chloride in the electrolysis being sufficient to maintain the rate of addition of sodium chlorate to the reaction medium. The gaseous mixture of chlorine dioxide, chlorine and steam was continuously removed from the reaction zone. The steam was continuously cooled and chlorine was continuously separated from the mixture. An aqueous solution of the chlorine dioxide was continuously produced and was continuously removed. A chlorine reactant stream was continuously formed by mixing feed chlorine with the chlorine separated from the mixture. The chlorine in the reactant stream was continuously reacted with the substantially stoichiometric quantity of gaseous hydrogen to form hydrogen chloride in accordance with the equation: EQU H.sub.2 +Cl.sub.2 .fwdarw.2HCl
the hydrogen which was reacting with the chlorine was provided by the hydrogen formed in the production of the sodium chlorate. That hydrogen chloride was continuously fed in the reaction medium in gaseous form or as hydrochloric acid, the rate of production of hydrogen chloride, and hence the rate of addition of feed chlorine to the system being sufficient to maintain the rate of addition of hydrogen chloride to the reaction medium. Excess hydrogen was continuously discharged from the system. A problem with this patented process is the increased cost due to the additional equipment necessary to recycle the precipitated salts.
U.S. Pat. No. 2,484,402 patented Oct. 11, 1949 to G. A. Day et al, provided a cyclic process which involved reacting solutions of chlorates with hydrochloric acid, the acid being supplied in an amount substantially less than the equivalent stoichiometric ratio of H+/ClO.sub.3.sup.- =2 of the chlorine dioxide producing reaction, thereby decomposing at any one time only a fraction of the available chlorate, the decomposition thus proceeding at a particularly rapid rate. The chlorate content of the partially spent solution was enriched, as for example, by feeding it to an electrolytic chlorate cell. The fortified solution was returned to the reaction chamber to treat it again with a stoichiometrically insufficient amount of acid. This cycle was repeated, whereby substantially all the chlorate supplied was eventually efficiently decomposed, producing mixtures of chlorine dioxide and chlorine containing high proportions of chlorine dioxide.
U.S. Pat. No. 3,404,952 patented Oct. 8, 1968 by G. O. Westerlund, provided a continuous process for the production of chlorine dioxide. The process included effecting electrolysis of an aqueous solution of a metal chloride, in order to form an aqueous solution of a metal chlorate, and gaseous hydrogen. The gaseous hydrogen was reacted with gaseous chlorine, in order to form gaseous hydrogen chloride. The aqueous solution of metal chlorate was reacted with the gaseous hydrogen chloride in order to form an aqueous solution of metal chloride, which was recycled to the first step and an aqueous solution of chloric acid. The aqueous solution of chloric acid was reacted with the gaseous hydrogen chloride in order to form chlorine dioxide, which was recovered water, and gaseous chlorine which was recycled to the second step.
U.S. Pat. No. 3,524,728 patented Aug. 18, 1970 by G. O. Westerlund, provided a closed cycle system for the generation of chlorine dioxide. The system included an electrolytic apparatus for the generation of an aqueous solution of chlorate. The electrolytic apparatus included a liquor inlet, a liquor outlet and a gas outlet. Generator apparatus was provided for generating chlorine dioxide from the chlorate solution and hydrogen chloride. That apparatus included a liquor inlet, a liquor outlet, a gas inlet and a gas outlet. The liquor outlet of the electrolytic apparatus was connected to the liquor inlet of the generator apparatus. Combustion apparatus was provided for the conversion of the cell gases from the electrolytic apparatus to hydrogen chloride. The combustion apparatus included a gas inlet and a gas outlet. The gas outlet of the electrolytic apparatus was connected to a gas inlet of the apparatus. The gas outlet of the combustion apparatus was connected to the gas inlet of the generator apparatus. A separator was provided for separating chlorine dioxide from gaseous chlorine, such separator including a gas inlet, a gas outlet, a liquid inlet and a product outlet. The gas outlet of the generator apparatus was connected to the gas inlet of the separator. The gas outlet of the separator was connected to the gas inlet of the combustion apparatus. The gas outlet of the separator was connected to the gas inlet of the generator apparatus. In this integrated plant a crystallizing generator is used. Solid sodium chloride crystals were generated in the crystallizing generator. The solid sodium chloride crystals were separated from the generator solution and were forwarded to the electrolysis area. Prior to electrolysis, the solid sodium chloride is dissolved in water and then was returned to the cells. A problem with this patented process is that additional equipment is required to recycle the salts.
U.S. Pat. No. 3,607,027 patented Sep. 21, 1971 by G. O. Westerlund provided an improved process for preparing chlorine dioxide wherein the bulk of the reactants were internally produced. The reactants included an aqueous solution of an inorganic chlorate and an aqueous hydrochloric acid. The aqueous solution of the inorganic chlorate was produced by electrolyzing an aqueous solution of an inorganic chloride. The aqueous hydrochloric acid was produced from hydrogen gas, which was a by-product of the electrolysis reaction by which the inorganic chloride was converted to the inorganic chlorate, and chlorine gas, followed by dissolving in water. In reacting the aqueous inorganic chlorate with the aqueous hydrochloric acid, both gaseous chlorine dioxide and gaseous chlorine were formed. The gaseous chlorine was separated from the gaseous chlorine dioxide. A portion of the so-separated gaseous chlorine was reacted with the hydrogen to form gaseous hydrogen chloride. The gaseous hydrogen chloride was used as a reactant, either by dissolving it in a stoichiometrically-insufficient quantity of water, thereby to provide an aqueous solution of hydrochloric acid and free hydrogen chloride gas, or by dissolving it in water containing absorbed chlorine gas.
U.S. Pat. No. 3,929,974 patented Dec. 30, 1975 by J. D. Winfield provided a process for the production of chlorine dioxide by continuously feeding an aqueous solution of an alkali metal chlorate and hydrochloric acid to a reaction zone to maintain an aqueous reaction medium in the reaction zone containing an alkali metal chlorate and hydrochloric acid. Chlorine dioxide and chlorine were continuously formed by reaction between the alkali metal chlorate and the hydrochloric acid in the reaction zone. The reaction zone was continuously maintained under a reduced pressure. The medium in the reaction zone was continuously maintained at its boiling point to evaporate water from the reaction medium continuously to form a gaseous phase in the reaction zone consisting of a mixture of chlorine dioxide, chlorine and water vapour, and to deposit alkali metal chloride in the reaction zone. The gaseous phase mixture was continuously conducted out of the reaction zone. Chlorine dioxide was continuously recovered from the mixture. Deposited alkali metal chloride was removed from the reaction zone. An aqueous solution was formed from the removed alkali metal chloride. That aqueous solution was continuously electrolyzed to convert the alkali metal chloride at least partially to alkali metal chlorate and to generate hydrogen gas. The alkali metal chlorate-containing solution was continuously fed to the reaction zone. The hydrogen gas was continuously reacted with chlorine gas to generate hydrogen chloride. Hydrochloric acid was continuously formed from the hydrogen chloride. That hydrochloric acid was continuously fed to the reaction zone. A problem with this patented process is the increased cost due to the additional equipment necessary to recycle the salts.
U.S. Pat. No. 4,543,243 patented Sep. 24, 1985 by H. Frohler, provided a process for the continuous production of chlorine dioxide. An alkali metal chlorate solution containing alkali chloride was reacted with hydrochloric acid to form a gaseous mixture of ClO.sub.2 and Cl.sub.2 and an alkali metal chloride solution depleted of chlorate, in a cascade reactor under an under reduced pressure at the outlet of the cascade reactor. A flow of air was introduced into the reactor to maintain a partial pressure of ClO.sub.2. The gaseous mixture was passed into a separation column where ClO.sub.2 was washed out with water to form a solution of ClO.sub.2. The Cl.sub.2 remaining after the ClO.sub.2 had been washed out of the gaseous mixture was reacted in an HCl Synthesis furnace with hydrogen which was cathodically-formed in a chlorate-electrolysis plant, along with externally produced Cl.sub.2 to give hydrochloric acid. That hydrochloric acid was introduced into the cascade reactor. The alkali metal chloride solution depleted of chlorate was withdrawn from the outlet of the reactor and was introduced into the chlorate-electrolysis plant to form chlorate and hydrogen. The chlorate solution was then recycled into the reactor and the hydrogen is introduced into the HCl-synthesis furnace.
In addition to the above patents, which related to the entire system and process for producing ClO.sub.2, several patents directed to chlorine dioxide generator designs have been provided. Many of these are based on the same chemical process as described above. Many also provide for the generation of chlorine dioxide using more than one generator.
U.S. Pat. No. 2,664,341 patented Dec. 29, 1953 by E. E. Kesting provided a system in which the chlorate and hydrochloric acid solutions were caused to flow through a series of consecutive reaction vessels staggered in height, one behind the other, arranged either in the form of a cascade, or as a column. In the opposite direction, a stream of inert gas was forced or drawn through the reaction vessels. The inert gas was caused to flow through the liquid by means of gas inlet pipes extending to the bottom of the vessels. The uppermost vessel was adjusted to the lowest temperature, while the lower-most vessel had the highest temperature. Under these conditions, the current of gas flowing through the apparatus had the lowest concentration of chlorine dioxide in the vessel having the highest temperature, while the chlorine dioxide concentration increased in the vessels above the lowermost. The temperature gradually becomes lower, and finally the highest chlorine dioxide concentration was obtained in the uppermost vessel where the lowest temperature prevailed.
If the lowermost vessel was heated to a given temperature, the gas leaving such vessel had a water vapour tension corresponding to the temperature. If this gas-steam mixture now entered into the next higher vessel of the series, it came into a thermal equilibrium with the contents of that vessel, i.e., it heated the vessel and its contents. From there, the gas current passed into the next higher vessel, heating the latter, and this was continued throughout the entire series of vessels. In this way, a temperature gradient was obtained in the vessels which automatically established itself, and which was dependent on the quantity of liquid, air and heat fed. If the reaction liquid was heated in this manner from vessel to vessel to a higher temperature, the result was that the conversion of chlorate and acid to chlorine dioxide was very complete, and took place with the best yield. Despite the high temperatures employed, there was said to be no danger of explosion. Furthermore, it was possible to maintain a desired given temperature gradient in the series of vessels, it being merely necessary to control the temperature at one place, for example at the place where the heat is introduced. All other temperatures were then automatically adjusted.
This patent, then, taught a method for maximizing the efficiency of the chlorine dioxide generator while depleting hydrogen chloride to acceptable levels. As described above, this method was to mix hydrochloric acid and sodium chlorate brine in a vessel at a low temperature and then to pass the liquid mixture through a series of vessels, with each successive vessel at a higher temperature. In this process, most of the chlorine dioxide was generated at a low temperature where efficiency was highest. Then, as the hydrogen chloride was depleted, the solution would cascade to a warmer vessel. Because each vessel was operated isothermally, temperature changes from vessel to vessel were discrete. In the warmer vessel, the effect of higher temperature (which increases reaction speed) would offset somewhat the effect of diminished concentration (which slows reaction speed). This allowed for high efficiencies while depleting hydrogen chloride concentration. The chlorine dioxide and chlorine generated was diluted and swept through the generator cascade and out of the generating system by an inert gas stream of air or nitrogen.
One problem with this system was the capital cost incurred by a multiple vessel design. Another problem of the design was the use of a diluent gas that had to be handled by down-stream equipment, thereby making chlorine recycle expensive and the separation of chlorine and chlorine dioxide problematical.
U.S. Pat. No. 3,502,443 patented Mar. 24, 1970 by G. O. Westerlund provided a unitary reaction apparatus. The apparatus included a vertically-disposed, primary reactor, the primary reactor including an upper, inflow liquor recirculatory zone adjacent the top thereof, a lower, outflow, liquor recirculatory zone adjacent the bottom thereof, a lower liquor outlet zone, and an upper gas outlet zone. The primary reactor being adapted to carry out at least an initial part of a designated chemical reaction. The gas zone was provided with a frangible explosion cover. A combined liquor circulating and fresh reactant feed inlet was attached thereto and communicated between the upper liquor recirculatory zone and the lower liquor recirculatory zone. A vertically-disposed secondary reactor was provided, the secondary reactor including a lower liquor inlet zone, an upper liquor outlet zone, and an upper gas outlet zone. That gas zone was also provided with a frangible explosion cover. The secondary reactor was attached to the primary reactor and communicated therewith by means of a connection from the lower liquor outlet zone of the primary reactor to the lower liquor inlet zone of the secondary reactor. The secondary reactor was adapted to carry out at least a terminal part of the designated chemical reaction. A primary gas collection zone was provided adjacent the gas outlet zone of the primary reactor, the primary gas collection zone leading to the gas outlet zone. Gas withdrawal and collection means were connected to the upper gas outlet zone. A secondary gas collection zone was provided adjacent the gas outlet zone of the secondary reactor, the secondary gas collection zone leading to the gas outlet zone of the secondary reactor. Gas withdrawal and collection means were connected to the upper gas outlet zone of the secondary reactor. Liquor outlet means were connected to the upper liquor outlet zone of the secondary reactor for withdrawal and collection of liquor, but not of gases, from the unitary reaction vessel. This system suffered the disadvantage that no provision was made to optimize the ratio of ClO.sub.2 to Cl.sub.2 initially produced.
U.S. Pat. No. 4,396,592 patented Aug. 2, 1983 by J. Combroux provided a system for the production of chlorine dioxide. The system included a primary reaction zone for the introduction of an alkali metal chlorate in aqueous solution and hydrogen chloride and an inert gas at a specified temperature for a specified residence time. The system included secondary reaction zones where the reaction products, which were removed from the primary reaction zone, were filled and emptied in sequence, the temperature in each of the secondary reaction zones being equal to, or at most a specified temperature greater than, that in the primary reaction zone and for a specified residence time. This design used the technique of increasing the temperature of the acidified brine to achieve high efficiency while attempting to assure substantially complete conversion of the hydrogen chloride. This system suffered the disadvantage that no provision was made to optimize the ratio of ClO.sub.2 to Cl.sub.2 initially produced.
Previously referred to U.S. Pat. No. 4,543,243 by Hans Frohler et al, taught a cascading generator, wherein the problem of the cost of multiple vessels was addressed. The generator design taught in this patent was a single vessel that was divided into sections by horizontal plates, with each plate one above the other so that the reactor was divided into a plurality of vertically-stacked sections. Each section was held at isothermal conditions, with the top section being the coolest section and the bottom section being the hottest section. Hydrochloric acid and strong brine were introduced into the top section of the reactor. The hydrochloric acid and strong brine mixture then passed downward through each section and exited the bottom of the reactor. In passing, the hydrochloric acid and strong brine mixture experienced discrete increases in temperature. Air was blown into one of the lower sections to dilute and strip the chlorine dioxide gas that was generated from the hydrochloric acid and strong brine mixture. This design used the technique of increasing the temperature of the acidified brine to achieve high efficiency while attempting to assure substantially complete conversion of the hydrogen chloride. However, the problem of the diluent air gas, as discussed previously, remained.
Previously referred to U.S. Pat. No. 4,851,198 by Karl Lohrberg taught another cascading reactor. This design was similar to the design described in U.S. Pat. No. 4,543,243, except that bubble cap trays were specified as a means to divide the reactor into sections. This patent also made provision for multiple acid injection ports, so that hydrochloric acid may be injected in more than one section. This design used the technique of increasing the temperature of the acidified brine to achieve high efficiency while attempting to assure substantially-complete conversion of the hydrogen chloride. However, the problem of the diluent gas, as discussed previously remained. The means by which the temperature of each section was positively controlled was not identified.
U.S. Pat. No. 4,938,944 patented Jul. 3, 1990 by Rainer Dworak et al provided a cascading generator. This design was a single vessel that was divided into vertically-stacked sections, and was very similar to the generator design taught in U.S. Pat. No. 4,851,198 except that a separation chamber was attached. The separation chamber was used to separate steam from the hot depleted brine that was produced from the bottom section of the generator. The steam from the separation chamber was used indirectly to heat sections of the generator. This reactor was air or nitrogen swept. The reactor was compartmentalized and the compartments were indirectly heated. This design used the technique of increasing the temperature of the acidified brine to achieve high efficiency while attempting to assure substantially-complete conversion of the hydrogen chloride. However, the problem of the diluent sweeping gas, as discussed previously remained.
Other patents provided additional designs for primary chlorine dioxide generators and their uses.
U.S. Pat. No. 4,075,308 patented Feb. 21, 1978 by W. H. Rapson et al provided a system of producing chlorine dioxide from sodium chlorate. The system included a reaction zone for the reaction of an aqueous reaction medium containing dissolved quantities of sodium chlorate, sodium chloride and hydrochloric acid, the aqueous reaction medium being substantially saturated with sodium chloride. The apparatus also included means for maintaining the reaction medium at its boiling point at the absolute pressure therein and for continuously maintaining the reaction zone under a sub-atmospheric pressure sufficient to maintain the reaction medium at its boiling point. The apparatus further included means for continuously removing, from the reaction zone, a gaseous mixture of chlorine dioxide, chlorine and steam wherein the volume ratio of steam to chlorine dioxide was greater than that below which substantial decomposition of chlorine dioxide occurred. The apparatus also included means for continuously precipitating the generated sodium chloride from the reaction medium in the reaction zone and for removing the deposited sodium chloride from the reaction zone. This design used the technique of increasing the temperature of the acidified brine to achieve high efficiency while attempting to assure substantially complete conversion of the hydrogen chloride. However, the problem of the diluent gas, as discussed previously remained. In addition, the problem of handling the slurry of sodium chloride remained.
U.S. Pat. No. 4,137,296 patented January 1974 by D. N. Glew et al provided a generator which also functioned as a crystallizer. The generator itself was simple in design, and thus had a low installed cost. However, this patent suffered the disadvantage that, to put this generator into an integrated plant, required some additional support equipment to handle slurried sodium chloride, which offset the low cost of the generator and increases the complexity of the integrated plant.
Many patents also exist for secondary chlorine dioxide generators.
Canadian Patent No. 1,136,378 patented Nov. 30, 1982 provided a system for the production of an aqueous solution of chlorine dioxide. The apparatus included an upright gas-liquid contact reaction zone. Means were provided for feeding a solution of sodium chlorate to the upper end thereof and for feeding a stream of gaseous sulphur dioxide to a lower end of the upright gas-liquid contact reaction zone. Means were provided for counter-currently contacting downwardly-flowing sodium chlorate solution and upwardly-flowing gaseous sulphur dioxide in the reaction zone to cause reaction therebetween. Means were provided for subjecting the reaction zone to a sub-atmospheric pressure which was greater than the pressure at which the reaction medium boiled. Means were provided to withdraw the gaseous chlorine dioxide therefrom at the upper end of the reaction zone. Means were provided for contacting the withdrawn gaseous chlorine dioxide with water to dissolve the chlorine dioxide therein at a flow rate of water sufficient to form an aqueous chlorine dioxide solution. This design again used the technique of increasing the temperature of the acidified brine to attempt to achieve high efficiency while attempting to assure substantially-complete conversion of the hydrogen chloride. Nevertheless, a problem with this system is that the generator system is not adaptable to an integrated design.
Canadian Patent No. 1,295,477 patented Oct. 22, 1985 provided a system of producing chlorine dioxide. The system included a reaction zone and means for continuously feeding an aqueous solution of sodium chlorate to the aqueous reaction medium in sufficient quantity to maintain a specified concentration of sodium chlorate in the reaction medium. Means were provided for continuously feeding hydrochloric acid or hydrogen chloride gas to the aqueous reaction medium. Means were provided for continuously maintaining the actual hydrogen ion concentration in the reaction medium in a specified range. Means were provided for continuously maintaining the reaction medium at its boiling point at the absolute pressure therein. Means were provided for continuously maintaining the reaction zone under a sub-atmospheric pressure sufficient to maintain the reaction medium at its boiling point. Means were provided for continuously removing from the reaction zone, a gaseous mixture of the generated chlorine dioxide, and chlorine gases and steam. Means were provided for continuously depositing the generated sodium chloride from the reaction medium in the reaction zone. Means were provided for continuously maintaining the volume of liquid in the reaction zone substantially constant. This design again uses the technique of increasing the temperature of the acidified brine to achieve high efficiency while assuring substantially complete conversion of the hydrogen chloride. However, the problem of the diluent gas, as discussed previously remains. The means by which the temperature of each section was discretely controlled is not identified within this patent. This system also suffered the disadvantage that provision had to be made to remove the precipitated salts and the salt slurry.
U.S. Pat. No. 3,895,100 patented Jul. 15, 1975 by G. Cowley provided a system for producing chlorine dioxide. The system included a reaction zone which was provided with means for maintenance at reduced pressure and means for evaporating water from the reaction medium at the boiling point thereof while the reaction medium was maintained under that reduced pressure. Means were provided for continuously removing, from the reaction zone, a gaseous mixture consisting of the generated chlorine dioxide and chlorine and the evaporated water, and for continuously recovering chlorine dioxide from the gaseous mixture. Means were provided for continuously depositing, in the reaction zone, an alkali metal salt of the anion of the strong acid, and for continuously removing, from the reaction zone, a slurry containing deposited alkali metal salt and a part of the reaction medium. Means were provided for heating the recycle mixture to the boiling point of the reaction medium at the prevailing absolute pressure in the reaction zone. Means were provided for accelerating the heated mixture to establish a back pressure to exceed the difference in saturation vapour pressure of the heated mixture and of the reaction medium. Means were provided for adding strong mineral acid to the accelerated heated mixture at the maximum velocity of the mixture resulting from the acceleration to provide a feed material for the reaction zone. Means were provided for allowing gaseous material in the feed material to expand while maintaining a low rate of acceleration. Means were provided for feeding the expanded feed mixture to the reaction zone at a level above the liquid level in the reaction zone. Means were provided for maintaining the level of liquid in the reaction zone substantially constant. This system suffered the disadvantage of having to provide means for the handling and removal of the salt slurry.
U.S. Pat. No. 3,929,974 patented Dec. 30, 1975 by J. D. Winfield provided a system for the production of chlorine dioxide. The system included a reaction zone having means for continuously maintaining the reaction zone under a reduced pressure. Means were provided for continuously maintaining the reaction medium at its boiling point to evaporate water from the reaction medium to form continuously a gaseous mixture of chlorine dioxide, chlorine and water vapour, and to deposit alkali metal chloride in the reaction zone. Means were provided for continuously conducting the gaseous phase mixture out of the reaction zone, and for continuously recovering chlorine dioxide from the mixture. Means were provided for removing deposited alkali metal chloride from the reaction zone, and for forming an aqueous solution from the removed alkali metal chloride. Means were provided for continuously electrolyzing the aqueous solution from the removed alkali metal chloride to convert the alkali metal chloride at least partially to alkali metal chlorate and to generate hydrogen gas. Means were provided for continuously feeding at least the alkali metal chlorate content of the alkali metal chlorate-containing solution to the reaction zone. Means were provided for continuously reacting at least part of the hydrogen gas with chlorine gas to generate hydrogen chloride. Means were provided for continuously forming hydrochloric acid from the hydrogen chloride, and for continuously feeding at least part of the hydrochloric acid to the reaction zone. This system suffered the disadvantage that means has to be provided for the handling and removal of the deposited slat and the salt slurry.
U.S. Pat. No. 3,975,505 patented Aug. 17, 1976 by W. A. Fuller provided a system for continuously generating a mixture containing chlorine dioxide, chlorine and a neutral alkali metal salt. The system included a single vessel generator-evaporator-crystallizer. Means were provided for maintaining the temperature at a specified level. Means were provided for subjecting the reaction solution to a vacuum to effect evaporation of water vapour. Means were provided for withdrawing chlorine dioxide and chlorine produced by the reaction solution in admixture with the water vapour. Means were provided for crystallizing the neutral alkali metal salt of the mineral acid within the generator-evaporator-crystallizer and for withdrawing therefrom in the form of an aqueous slurry containing minor amounts of chlorate, chloride and acid values. Means were also provided for continuously passing the slurry containing neutral alkali metal salt crystals produced in the generator-evaporator-crystallizer into the top of a separatory column, in a downward flow. Means were provided for countercurrently passing a stream of hot water continuously upwardly through that column at a rate sufficient to effect washing of the downwardly flowing crystals, and for continuously and substantially completely returning chlorate, chloride and acid values recovered therefrom to the generator-evaporator-crystallizer. Means were provided for continuously removing an aqueous slurry of substantially pure neutral alkali metal sulfate salt crystals from the bottom of the separatory column. This system suffered the disadvantage that provisions had to be made to remove the salt slurry.
U.S. Pat. No. 4,543,243 patented Sep. 24, 1985 by H. Frehler et al provided an apparatus for carrying out a process for the continuous production of chlorine dioxide. The system included a cascade reactor having means for the introduction of air. Means were provided for providing a reduced pressure within the reactor. Means were provided for separating chlorine gas separated off from the reaction product and for reaction of such gas with cathodically-formed hydrogen, along with the introduction of external chlorine gas, to give the required amount of hydrochloric acid. The apparatus included at least one chlorate electrolysis plant, a cascade reactor, a separation column and a hydrochloric acid synthesis furnace, which were connected with one another. A valve was provided in the air inlet pipe of the reactor, which was regulated corresponding to the pressure at the product outlet of the reactor. The problem of the diluent gas, as discussed previously remains.
U.S. Pat. No. 4,938,944 patented Jul. 3, 1990 by R. Divorak provided a reactor for the production of a gaseous mixture containing chlorine dioxide and chlorine, by reacting alkali metal chlorate in an aqueous solution of hydrochloric acid. The reactor included a plurality of superimposed reaction levels which were traversed by the solution from top to bottom. In the lower portion of the reactor, the solution in which chlorate and acid had been depleted was reboiled in a reboiling chamber by an indirect heating at a temperature in the range of from about 100.degree. to about 110.degree. C. The depleted solution was conducted from the reboiling chamber to a pressure chamber, in which a pressure of at least 1.2 bar was maintained. In the pressure chamber, the solution was reboiled at temperatures from about 110.degree. to about 150.degree. C. and the vapours formed by the reboiling in the pressure chamber were conducted through the reboiling chamber for an indirect heating therein. This design used the technique of increasing the temperature of the acidified brine to achieve high efficiency while attempting to assure substantially complete conversion of the hydrogen chloride. However, the provision of pressure chambers brings about problems of costly reactor construction.